According to present laboratory practice, when it is desired to microscopically view a liquid, small samples of the liquid are placed on microscope slides in a layer as uniformly thin as a laboratory technician can achieve. While many variations of centrifuge devices for creating specimen slides for microscopic examination presently exist, problems inherent in their use have caused many laboratories, schools and research facilities to rely on the manual wedge technique of preparing liquids for microscopic examination. In this technique, two slides are used. On the first slide the sample, for example blood, is placed as a large drop. The other slide is angled toward the sample-bearing surface of the first slide until the edge of the second slide touches the sample. The second slide is then moved along the length of the first slide, literally smearing the sample over the entire surface of the first slide.
While fairly inexpensive, the manual wedge technique has many drawbacks. Only one slide can be prepared at one time, taking up valuable laboratory technician time. Excessive sample amounts are used to create the smear. Laboratory slides are wasted since only one of the two slides used in the technique is available for microscopic examination. Moreover, the number of cells per microscopic field viewable by this technique is very few.
Usually the sample on a specimen slide prepared using the wedge technique is thick. When the liquid contains cells, this hampers accurate evaluation of the morphology of the cells, which often is a critical factor in the diagnosis of diseases. As an example, in the diagnosis of leukemia it is critical to determine the number, exact type and derivation of leucocytes in the blood. Thick smears of the blood sample on a slide for microscopic examination can cause distortion in the characteristic shape of these cells, causing one cell type to be mistaken for the other. This results in false negative and false positive diagnoses of leukemia.
Additionally, the cells in the samples produced by the method are very few in number and very widely dispersed over almost the entire surface of the slide. Such a wide field for observation also creates inaccuracy and delays in performing differential counts of cell types, since the observer must constantly change the microscopic filed in all directions to examine the cells on the entire slide. Moreover, such wide dispersal also makes it almost impossible to count and identify cells, i.e., blood cells, bacterial cells, or virus particles, in patients with very low numbers of such cells. Because this procedure is slow and tedious, and because it is easy to mistakenly recount, or omit to count, a field many laboratory workers will only count a few fields rather than the entire slide. Thus, the relative number of cell types per amount of sample is frequently inaccurate, another contributing factor to misdiagnosis of the disease in question.
A further disadvantage is the relatively large amount of specimen sample required to create the specimen slide. This disadvantage is apparent in cases where the liquid in question is a biological body fluid such as blood, spinal fluid, or serum, and when the patient from whom the specimen is to be taken is either very young or very old and frail.
Methods for the production of specimen slides of liquid samples utilizing centrifugation were developed in attempts to overcome the disadvantages of the wedge technique. The prior art devices used for practicing such methods are revealed in the following patents, in which the samples are generally cell-containing biological liquids.
Johnson U.S. Pat. No. 4,294,866--Oct. 13, 1981, PA1 Bacus U.S. Pat. No. 4,209,548--June 24, 1980, PA1 Barger et al. U.S. Pat. No. 4,108,109--Aug. 22, 1978, PA1 Mikat U.S. Pat. No. 3,870,789--Mar. 11, 1975, PA1 Staunton U.S. Pat. No. 3,705,048--Dec. 5, 1972, PA1 Preston et al. U.S. Pat. No. 3,577,267--May 4, 1971.
Holroyd et al. U.S. Pat. No. 4,197,329--Apr. 8, 1980,
The common concept upon which the methods disclosed in the above-referenced patents and devices operate is that when a slide or other sample receiving surface on which is placed a small amount of liquid is rotated with its sample-receiving surface normal to a vertical axis of rotation, centrifugal force on the sample throws off excess liquid and spreads the sample radially, forming a layer which covers approximately the entire surface of the slide.
While this concept of centrifugation solved the wedge technique problems of technician time and an excessively thick specimen layer, they also introduced other problems of their own, as well as sharing problems with the wedge technique method. These inherent problems include the danger of contamination from infectious liquids due to the radial expulsion of excess liquid from the sample placed on the slide; the chances for some distortion or disturbance of cell particle relationships due to the speed and length of time at which the slides are spun; the use of one slide for each sample; the time factor involved in subjecting only one slide at a time to centrifugation; the required sample sizes which often are difficult to obtain from elderly or infant patients; the width of the resulting smear covering the entire surface; and the current production costs of the spinning apparatus.
The danger of contamination of the surrounding atmosphere, the spinning device, the smear itself, and the distortion of the cells in the smear result from the principle of centrifugation applied in present practice. The usual size of the liquid samples required for centrifugation methods, while less than that generally needed for the wedge technique, allows for the radial expulsion of the upper excess layers during spinning to leave behind only a thin radially dispersed smear which dries as the spinner slows down. Because the centrigual force created on the slide surface spinning at a right angle to the vertical axis of rotation spreads the sample in all directions, various cell smear devices must incorporate waste capturing means such as wells to the spinner, thus increasing the complexity of the machine and its cost of production.
Distortion of cells, especially blood or bacterial cells, will result when residual liquid, upon drying, tends by surface tension to flatten the cells. In addition, the problem of creating a wide field for observation and differential counting still exists, creating the same basis for misdiagnosis as does the wedge technique.
There exists, therefore, a need for an improved method and apparatus to increase the efficiency and accuracy of preparation of liquid specimen slides for microscopic examination, particularly where the liquid is a biological cell-containing liquid, which can overcome the problems evident in the present methods of practice.